In most countries, access to radio frequency spectrum is tightly regulated through government agencies such as the Federal Communications Commission (FCC) in the United States or the European Commission in the European Union. Like any other natural resource, the frequencies that make up the radio spectrum need to be shared among its users. Parts of the radio spectrum, so-called bands, are thus either licensed to individual users, such as mobile operators, or shared among many users as is the case with WiFi or Bluetooth which operate in unlicensed bands. In addition, certain hybrid models exist where licensed spectrum is granted to a primary user, for instance for naval radar applications, who has the highest priority. In addition, secondary users are allowed to use the licensed band during periods of inactivity during which the primary user does not transmit waveforms in the band under consideration. These secondary users may have a different priority. For example, a given frequency band licensed to a primary user could be used by the public safety community for mission critical communications. In this case, commercial users could be allowed to use such a band and only if both the primary and the secondary user of higher priority, i.e. a public safety user, do not occupy the band. Such policy based spectrum usage is sometimes referred to as Authorized Shared Access (ASA). From this perspective, there is no need to distinguish between unlicensed and authorized shared access as the same techniques can be used to ensure fairness and policy compliance whenever a band is used by many users.
In the above example of authorized shared access, spectrum sharing can be facilitated by dynamic schemes, sometimes referred to as listen-before-talk (LBT) schemes, as well as semi-static schemes, such as through geolocation databases (GLDB). Such databases, for example, can map frequency usage of certain bands to geographic areas or times of day. Due to the time it takes to update and propagate these databases to all participating users, they cannot change dynamically. As the name suggests, LBT schemes are more dynamic and do not rely on semi-statically configured databases. Rather, a secondary user has to ensure that the primary user or other user of equal priority is not interfered with by its transmission. Two well-known examples are radar avoidance and Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) in IEEE 802.11 Wireless Local Area Networks (WLANs). The former applies to the case when there is a primary user, to which a secondary user must grant priority. Since the secondary must cease transmission when it detects a military, meteorological or automotive radar waveform, it is often referred to as Dynamic Frequency Selection (DFS). In other words, the secondary user vacates a given band or channel (channels are further subdivisions of bands) upon detection of a primary user and tries to transmit on a different band or channel giving rise to the name dynamic frequency selection. Similarly, in the case of CSMA/CA, when the transmitter detects an ongoing transmission of equal priority, it chooses not to transmit in order to try again at a later point in time. Hence the name carrier sense multiple access with collision avoidance. The two main differences between DFS and CSMA/CA, therefore, are the time scale at which the sensing occurs and the action the transmitter takes when an on-going transmission is detected. For example, a DFS transmitter will always have to switch channels/bands in order to vacate the current one for the primary user, whereas a CSMA/CA transmitter may or may not switch the channel. This is because in CSMA/CA the radio resources are shared among users of equal priority, and it is considered a multiple access scheme. With DFS, however, the primary user has higher priority. Consequently, in order to guarantee competitive latencies of CSMA/CA schemes, the carrier sensing (CS) and collision avoidance (CA) occurs in the order of tens of microseconds (μs) whereas DFS may take seconds.
CS/CA multiple access schemes stand in stark contrast to other common multiple access techniques such as Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA), or Orthogonal Frequency Division Multiple Access (OFDMA) due to the opportunistic random access nature by which the medium is shared. TDMA and FDMA in the Global System for Mobile Communications (GSM), CDMA in the Universal Mobile Telecommunications System (UMTS), and Orthogonal Frequency-Division Multiple Access (OFDMA) in the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE)) try to orthogonalize the available resources to share them among multiple users. Orthogonal operation, however, requires precise coordination through predefined rules or a dynamic scheduler which assigns resources to particular users for a given period of time in a given part of the radio frequency spectrum such that collisions are inherently prevented. This makes it particularly challenging to operate them in radio resources which are shared by means of CS/CA multiple access schemes, since users following those kinds of protocols and procedures would lose to users following predefined schedules or radio resource assignments according to their protocols and procedures when competing for the available radio resources.
In LTE a base station is known as an evolved NodeB (eNodeB/eNB) and is in full control of the Radio Resource Management (RRM) of a cell under its control. An Evolved Universal Terrestrial Radio Access Network (E-UTRAN) generally comprises of many eNodeBs each with its own RRM function. A subset of these eNodeBs can coordinate their RRM through the X2 Application Protocol (X2AP) which is defined on the X2 interface connecting two eNodeBs. Similarly, each eNodeB is connected to one or more of the Mobility Management Entities (MMEs) in the Core Network (CN) via the S1 interface on which the S1 Application Protocol (S1AP) is defined. The S1AP can be used for RRM coordination as well. RRM interfaces are an integral part of cellular communications as they allow important functions such as interference coordination, mobility, or even Self-Organizing Networks (SONs).
FIG. 1 is an exemplary wireless telecommunications network of the prior art. The illustrative telecommunications network includes primary eNodeB 110 operating in primary cell (PCell) 100 and eNodeBs 112, 114, 116, and 118 operating in secondary cells (SCell1 through SCell4) 102, 104, 106, and 108, respectively. A handset or other user equipment (UE) 120 is shown in communication with eNodeB 110 of PCell 100. UE 120 may also be in communication with one or more eNodeBs of the secondary cells. Here, SCell is a logical concept. For example, eNodeB 110 could operate a plurality of SCells 102 through 108.
In addition, eNodeB 110 is in control of the radio resources in its cell 100 by means of the Radio Resource Control (RRC) protocol as well as the multiple access of the users connected to its cell by means of the Medium Access Control (MAC) protocol. The RRC protocol, for instance, configures the carriers of which a User Equipment (UE) can transmit and receive data and up to five so-called Component Carriers (CCs) can be configured per UE in LTE Advanced (LTE-A). Similarly, the MAC protocol in conjunction with the RRC protocol controls how and when the UE can use the available radio resources to transmit or receive data on a configured carrier. LTE Release 10 introduces a feature called Carrier Aggregation in which a UE can be configured with one primary cell (PCell) and up to four secondary cells (SCells). A PCell can only be changed through a handover, whereas SCells are configured through RRC signaling. In particular, a UE is not expected to receive system information by decoding the Physical Broadcast Channel (PBCH) on a Secondary Component Carrier (SCC) or to monitor the common search space of an SCell to receive Physical Downlink Control Channels (PDCCHs) whose CRC is scrambled by the SI-RNTI in order to receive System Information (SI) on the Downlink Shared Channel (DI-SCH). Moreover, the UE may assume that the System Frame Number (SFN) on all SCCs is aligned with the SFN of the Primary Component Carrier (PCC).
CA does not define Radio Link Monitoring (RLM) of an SCell. As such, there is no specified means for the UE Physical layer (PHY) to indicate a Radio Link Failure (RLF) to the UE higher layers through the MAC layer. This is because in the Evolved Universal Terrestrial Radio Access (E-UTRA) one can always rely on the connectivity provided by the PCell which provides robustness through RLM and other fallback procedures. Alternatively, one may think of SCells as supplementary serving cells which can be activated in case additional capacity is needed for data communication with the UE. To this end, the MAC layer can activate configured SCells through a MAC Control Element (CE). An SCell activation can take between 8-30 ms depending on the synchronization status of the UE with respect to that SCC. An RRC reconfiguration of an SCell would take significantly longer, especially when the UE needs to perform an inter-frequency measurement. The eNodeB may thus configure a UE to periodically measure the Reference Signal Received Power (RSRP) of certain cells on certain carriers and to report the measurements either periodically or triggered through configurable offsets and thresholds. In the 3GPP Long Term Evolution this is achieved through RRC signaling of measurement objects and configurations. If measurements are readily available at the eNodeB, an RRC reconfiguration of an SCell or PCell can be dramatically reduced in latency from seconds to tens or hundreds of milliseconds. Note that while the eNodeB can only activate cells that are already configured as SCells, it can configure a UE to measure the RSRP on any cell. On the other hand, the eNodeB can use the measurement report of any cell to either activate a cell, as in the case of SCell activations, or to RRC reconfigure the UE to add/remove SCells or even to change the PCell.
Once a PCell or SCell is activated, the eNodeB MAC scheduler assigns downlink (DL) and uplink (UL) grants to a UE for downlink and uplink transmissions on the Physical Downlink Shared Channel (PDSCH) and Physical Uplink Shared Channel (PUSCH), respectively. In the downlink direction, a grant received in the Downlink Control Information (DCI) in subframe n schedules a corresponding PDSCH transmission in the same subframe whereas in the uplink, it schedules PUSCH transmissions in subframe n+k, where k>0 is determined through pre-specified rules.
It is worth reiterating that the E-UTRAN, in particular, the eNodeB, is in full control of all radio resources at least for UEs in RRC_CONNECTED mode and that, with the exception of the Physical Random Access Channel (PRACH), it controls all transmissions in both the uplink and downlink direction including resource assignment in time, frequency, or any other means such as CDMA as well as timing or power control of a transmission.
Even though the RRM function resides in the eNodeB, which in turn controls all radio resources through RRC, it relies on the UE to discover cells and report associated measurements. To this end, in LTE Releases 8 through 11, the eNodeB transmits a Primary Synchronization Signal (PSS), a Secondary Synchronization Signal (SSS), and a Cell-specific Reference Signal (CRS) in each radio frame. The PSS and SSS each occupy one OFDM symbol per half-frame whereas the CRS is transmitted in each subframe of a radio frame thus allowing a UE to discover and measure cells within a measurement window of 6 ms without a-priori knowing the timing of a given cell. Furthermore, to support inter-frequency measurements in Time Division Duplex (TDD) systems when the UL/DL configuration of a cell may not be known to a UE or to support measurement restrictions introduced in LTE Rel. 10 for the purpose of enhanced Inter-cell Interference Coordination (eICIC), a UE must be able to discover cells in just one subframe and potentially the DwPTS region of a special subframe. In order to facilitate energy savings and interference reduction, LTE Release 12 introduces “discovery bursts” comprising PSS, SSS, and CRS transmissions and, if configured, Channel State Information Reference Signals (CSI-RS) for transmission point (TP) identification in shared cell ID scenarios. For example, multiple TPs may share the same physical cell ID and may only be discerned by their respective CSI-RS resource element (RE) configuration. PSS, SSS, CRS and CSI-RS (if configured) make up the Discovery Reference Signals (DRS) and are transmitted during DRS occasions. DRS occasions are similar to LTE Release 9 Positioning Reference Signal (PRS) occasions in that they have a configured or specified length (i.e. number of subframes) and periodicity. Ideally, the length of a DRS occasion does not exceed the UE measurement window of 6 ms and could be as short as one subframe. Reasonable periodicities for DRS occasions are hundreds of milliseconds and DRS bursts can be thought of as beacons in other wireless communication systems such as CSMA/CA.